Question

If all your cells come from the original zygote that formed you, why aren’t all of them identical to each other?

Answer 1

Well genetically they are all the same!

Differences arise between them very early in development first in response to chemical signals from Mom and then in response to signals from other cells in the zygote. These differences reflect inactivation of various genes at various points of the DNA through epigenetic silencing . The untranscribed regions regions of DNA are literally folded up and packed away where they are inaccessible to transcription factors).

At the 8 cell phase, even though the cells are all still capable of turning into any of the different tissue types in the body, they are already starting down different paths - as signals reenforce those differences, their developmental scope becomes more and more limited until eventually they are fully differentiated somatic cells.